This invention relates to a high strength steel alloy, and more particularly, to a steel alloy which has improved mechanical properties (yield stength and ultimate tensile strength) and improved ambient and low temperature toughness (impact strength) following repeated thermal exposures which are related to applications of glass or enamel coatings thereto, and vitreous enamel-coated steel articles prepared therefrom. The alloys of the present invention are therefore particularly suited for use with coatings of vitreous enamels.
Steel alloys suitable for glassing not only must meet minimum standards for glassing prior to the glassing operation, but also must meet minimum mechanical and toughness standards after the glassing or enameling process which requires one or more elevated temperature exposures. Some of the prior art steel alloys have good mechanical and toughness properties after exposure to the heat necessary to glass the alloy, but they exhibit poor glassability. Other prior art steel alloys exhibit good glassing properties, but fail to meet the minimum mechanical and toughness standards required for a finished glassed steel product. Following the glassing operation which includes heating and cooling cycles for fusing of the glass coating material upon and to the steel substrates, it is desirable that the yield strength of the alloy at room temperature be at least about 30,000 psi, the ultimate tensile strength at room temperature be at least about 55,000 psi, and the Charpy "V" notch impact strength at -20.degree. F. be at least about 15 ft. lbs. Furthermore, to assist in attaining these properties, the Ac.sub.3 temperature (the completion of transformation on heating) should not exceed about 1600.degree. F. and the Ar.sub.1 temperature (the completion of transformation on cooling) should not extend below about 1200.degree. F. These temperature ranges are necessary in order to take advantage of grain refinement in the steel during a glassing cycle which is equal to the time-temperature cycle required to normalize the steel.
There are many steel alloys available for "glassing", which is the application of at least one coating or layer of glass or ceramic (vitreous enamel) upon the surface of the steel. These alloys which are described in the prior art, generally have excellent mechanical properties and toughness prior to the thermal exposure necessary to fuse and bond ceramic or glass coatings to the steel substrate, however, following one or more of the glassing cycles, that is, the application of the coating material under conditions which raise the temperature of the steel as high as about 1650.degree. F., the prior art steel alloys are characterized by numerous shortcomings and fail to meet the desired standards of one or all of these properties. In many cases there is a tendency of the glass coating to craze or crack during or after cooling the ceramic-metal composite from the firing temperature.
One of the major difficulties in enameling or coating of steel alloys with a vitreous or ceramic coating is the high transformation temperature of the alloys undergoing the coating. In U.S. Pat. No. 2,602,034 Eckel discloses a method of making enameling sheets from a slab of steel suitable for enameling stock. However, the alloy compositions disclosed by Eckel have low manganese content and no nickel, and accordingly, the Eckel alloy has a high transformation temperature and loses strength as a result of the thermal exposures accompanying the glassing operation, thereby resulting in a weakened steel substrate in the finished enameled product. Comstock et al in U.S. Pat. Nos. 2,495,835 and 2,495,836 disclose a steel alloy composition for vitreous enameling, the composition comprising, in general, below 1.0% titanium, not in excess of 0.15% carbon, not over 0.060% manganese, and not over 0.10% each of phosphorous, silicon, sulfur and the like. The alloys disclosed by Comstock et al are low in manganese, and nickel is only present in trace quantities. Accordingly, the alloy of Comstock et al has a high transformation temperature and fails to meet the desired strength requirements following repeated thermal exposure necessary for glassing the alloy.
In U.S. Pat. No. 2,303,064 Bernick et al disclose a titanium bearing steel alloy having vitreous enameling properties. The titanium must be present in the alloy in an amount at least four times that of the carbon, and by employing the titanium-containing steel of this type as a vitreous enameling base stock a single enamel covercoat of various kinds can be fused to the metal surface without the necessity of employing the usual ground coat thereto. A typical composition disclosed by Bernick et al is 0.045% carbon, 0.30% manganese, 0.01% phosphorus, 0.019% sulfur, 0.03% silicon, 0.018% aluminum and 0.34% titanium. However, the alloy in Bernick et al is low in manganese and contains no nickel. Accordingly, the Bernick et al alloy has a high transformation temperature and would not meet the minimum strength requirements after the thermal exposure necessary to apply the vitreous or ceramic coatings thereto. Titanium- and niobium-containing steel alloys are disclosed by Narita et al in the J. Iron and Steel Institute of Japan, 50, 43 (1964). The titanium-containing steel alloys of Narita et al consist of 0.14-0.39% titanium, 0.08-0.09% carbon, 0.28-0.31% silicon, 0.46-0.51% manganese, 0.006-0.009% phosphorus and 0.007% sulfur. The niobium-containing steel of Narita et al consists of 0.22-0.59% niobium, 0.08-0.09% carbon, 0.21-0.33% silicon, 0.48-0.51% manganese, 0.002-0.004% aluminum, 0.010-0.017% phosphorus and 0.007% sulfur. Although the alloy compositions of Narita et al exhibit a suitable strength before glassing, the strength of the steel following the thermal exposure necessary for the glassing technique would be too low to meet the desired values. The transformation temperature of the Narita et al alloy is too high to permit grain refinement during the exposure to heat. The alloy compositions of Narita et al do not contain nickel, and the manganese is sufficiently low so that the transformation temperatures of the alloys are high. The Narita et al alloy compositions are suitable for glassing, however, after the glassing operation, the yield strength and ultimate tensile strength are below the desired standards for applying glass or vitreous enamels to steels.